One teen stabbed another with a hair pick, an enraged mom pulled a knife on a school safety agent, and 12 weapons were seized at security checkpoints on a violent first day of classes in the Big Apple on Wednesday, officials and sources said.

The day of reading, writing and arrests began at 8:30 a.m., when one 17-year-old student gouged another with a hair pick as a long-simmering beef came to a head at Automotive High School in Brooklyn, cops and sources said.

The suspect — identified by a police source as Trevell Brown — jabbed his foe in the head and hand with the accessory during the 8:30 a.m. brawl outside the Greenpoint school, cops said.

The victim was treated at Woodhull Medical Center and is in stable condition.

Brown took off running after the attack but was nabbed by cops when he tried to slip back into the Bedford Avenue school a short time later, authorities said.

He still had the hair pick in his pants pocket, according to cops.

Brown — who had an ongoing feud with the victim, according to a school safety source — is charged with assault.

Brown’s hair pick was one of a dozen weapons seized at schools around the city on Wednesday, according to the school safety agents union.

The dangerous haul also included eight knives, two box cutters and a razor blade.

All of the confiscations occurred at schools equipped with metal detectors, sources said.

Elsewhere in Brooklyn, a mom trying to register her child at P.S. 149 on Sutter Avenue got into an argument with a school safety agent and pulled a knife, police said.

Agents were able to disarm and handcuff the woman before anyone was cut in the 9:50 a.m. fracas in East New York, authorities said.

Charges against the mom — whose name wasn’t immediately released — are pending.